Haomin Song

Broadband absorption engineering of hyperbolic metafilm patterns

We experimentally realize a patterned hyperbolic metafilm with engineered and freely tunable absorption band from near-IR to mid-IR spectral regions based on multilayered metal/dielectric hyperbolic metamaterial waveguide taper.

Dielectric-Grating-Coupled Surface Plasmon Resonance From the Back Side of the Metal Film for Ultrasensitive Sensing

We propose a dielectric grating that can launch surface plasmon resonance (SPR) modes efficiently on the other side of flat metal films, which is similar to the conventional prism coupling mechanism. Importantly, this structure can excite SPR under the normal incident light, which is particularly suitable for the integration with optical fiber tips. By launching the SPR mode near the wavelength of 1.55 μm with a very narrow resonance line width (~4 nm), this structure is promising for the development of high-performance portable, flexible, and real-time refractometric sensing applications.

Nanocavity Enhancement for Ultra‐Thin Film Optical Absorber

A fundamental strategy is developed to enhance the light-matter interaction of ultra-thin films based on a strong interference effect in planar nanocavities, and overcome the limitation between the optical absorption and film thickness of energy harvesting/conversion materials. This principle is quite general and is applied to explore the spectrally tunable absorption enhancement of various ultra-thin absorptive materials including 2D atomic monolayers.

Extremely Cost-Effective and Efficient Solar Vapor Generation under Nonconcentrated Illumination Using Thermally Isolated Black Paper

Passive solar vapor generation represents a promising and environmentally benign method of water purification/desalination. However, conventional solar steam generation techniques usually rely on costly and cumbersome optical concentration systems and have relatively low efficiency due to bulk heating of the entire liquid volume. Here, an efficient strategy using extremely low‐cost materials, i.e., carbon black (powder), hydrophilic porous paper, and expanded polystyrene foam is reported. Due to the excellent thermal insulation between the surface liquid and the bulk volume of the water and the suppressed radiative and convective losses from the absorber surface to the adjacent heated vapor, a record thermal efficiency of ≈88% is obtained under 1 sun without concentration, corresponding to the evaporation rate of 1.28 kg (m2 h)−1. When scaled up to a 100 cm2 array in a portable solar water still system and placed in an outdoor environment, the freshwater generation rate is 2.4 times of that of a leading commercial product. By simultaneously addressing both the need for high‐efficiency operation as well as production cost limitations, this system can provide an approach for individuals to purify water for personal needs, which is particularly suitable for undeveloped regions with limited/no access to electricity.

Refractive index engineering of metal-dielectric nanocomposite thin films for optical super absorber

Using metal-dielectric nanocomposite materials, we developed thin-film resonant and nonresonant absorbers with tunable absorption band. A compact double-side vertically graded metal-dielectric nanocomposite absorber was fabricated by gradually varying metal-dielectric nanocomposite ratios. The optical impedance of this metal-dielectric nanocomposite structure can be engineered to realize the antireflection characteristics. A broad-band and angle-insensitive super absorption over 81% was obtained in visible to near-infrared spectral region (i.e., 400 nm to 1100 nm), which is broader than recently reported plasmonic metamaterial absorbers in the similar spectral region.

Wide-Angle and Polarization-Insensitive Perfect Absorber for Organic Photovoltaic Layers

In this letter, we computationally explore a wide-angle and polarization-insensitive perfect absorber based on hybrid metal-dielectric-metal structures. By introducing an organic photovoltaic layer between top metallic nanopatterns and a continuous metal bottom plate, an enhanced angle-and polarization-insensitive absorption can be obtained in the spectral range 400-700 nm, which is promising to realize improved thin-film organic photovoltaic devices. The physical mechanism of the perfect absorber is explained theoretically and numerically by the critical coupling principle.

MoS2 monolayers on nanocavities: enhancement in light–matter interaction

Two-dimensional (2D) atomic crystals and van der Waals heterostructures constitute an emerging platform for developing new functional ultra-thin electronic and optoelectronic materials for novel energy-efficient devices. However, in most thin-film optical applications, there is a long-existing trade-off between the effectiveness of light–matter interactions and the thickness of semiconductor materials, especially when the materials are scaled down to atom thick dimensions. Consequently, enhancement strategies can introduce significant advances to these atomically thick materials and devices. Here we demonstrate enhanced absorption and photoluminescence generation from MoS2 monolayers coupled with a planar nanocavity. This nanocavity consists of an alumina nanolayer spacer sandwiched between monolayer MoS2 and an aluminum reflector, and can strongly enhance the light–matter interaction within the MoS2, increasing the exclusive absorption of monolayer MoS2 to nearly 70% at a wavelength of 450 nm. The nanocavity also modifies the spontaneous emission rate, providing an additional design freedom to control the interaction between light and 2D materials.

Single-crystalline germanium nanomembrane photodetectors on foreign nanocavities

High-yield, high throughput method creates nanomembrane photodetectors with unique optoelectronic properties. Miniaturization of optoelectronic devices offers tremendous performance gain. As the volume of photoactive material decreases, optoelectronic performance improves, including the operation speed, the signal-to-noise ratio, and the internal quantum efficiency. Over the past decades, researchers have managed to reduce the volume of photoactive materials in solar cells and photodetectors by orders of magnitude. However, two issues arise when one continues to thin down the photoactive layers to the nanometer scale (for example, <50 nm). First, light-matter interaction becomes weak, resulting in incomplete photon absorption and low quantum efficiency. Second, it is difficult to obtain ultrathin materials with single-crystalline quality. We introduce a method to overcome these two challenges simultaneously. It uses conventional bulk semiconductor wafers, such as Si, Ge, and GaAs, to realize single-crystalline films on foreign substrates that are designed for enhanced light-matter interaction. We use a high-yield and high-throughput method to demonstrate nanometer-thin photodetectors with significantly enhanced light absorption based on nanocavity interference mechanism. These single-crystalline nanomembrane photodetectors also exhibit unique optoelectronic properties, such as the strong field effect and spectral selectivity.

Artificial birefringent metallic planar structures for terahertz wave polarization manipulation

We propose an artificial birefringent terahertz (THz) device constructed by subwavelength L-shaped hole arrays on a single metallic layer. This structure is able to work as a polarizer when the incident frequency is between the cut-off frequencies of two eigenmodes. When the incident wave is beyond cut-off frequencies of these two modes, it can be designed as an efficient half- or quarter-wave plate with extraordinary transmission properties. A big effective index difference from 0.254 to 0.768 is obtained using a subwavelength-thick planar structure, which can reduce the thickness of the device to one tenth of conventional quartz birefringent crystals for THz waves.

Reversibly tunable coupled and decoupled super absorbing structures

We differentiate the spacer-dependent peak shift in coupled and decoupled super absorbing structures based on magnetic resonance and interference mechanism, respectively, which is experimentally validated by low-cost and large-area structures fabricated using lithography-free processes. The reversible real-time spectral tunability is then demonstrated by incorporating a thermally tunable polymeric spacer layer.